Speech recognition techniques that can be put to practical use in noisy environments have been more and more important. In a noisy environment, the accuracy of speech recognition is disadvantageously reduced by noise. Speech recognition is performed using a speech feature extracted from an input speech signal. A mel-frequency cepstrum coefficient (MFCC) is known as a type of speech feature. However, speech recognition using only the MFCC does not achieve sufficient noise tolerance. Hence, there has been a demand for a speech feature that enables the noise tolerance of the speech recognition to be improved.